Struct vs class as an STL function when using not2

Question

Studying STL, I wrote a simple program for testing functors and modifiers. My question is the difference using CLASS or STRUCT to write a functor and try to work with it using functional adapters. As far as I understand in C ++, the difference between CLASS and STRUCT is that in the latter case, members are public by default. This is also what I read many times in the answers on this site. Therefore, please explain to me why this short piece of code does not compile even if I declared all members (only overload functions ()) public when I try to use the not2 modifier. (I have not tried other modifiers, for example, binders)

#include <iostream>
#include <vector>
#include <functional>
#include <algorithm>
using namespace std;

template <class T>
void print  (T  i) {
    cout << " " << i;
}
// In the manual I read:
// "In C++, a structure is the same as a class except that its members are public by default."
//  So if I declare all members public it should work....

template <class T>
class mystruct : binary_function<T ,T ,bool> {
    public :
    bool operator() (T  i,T  j) const { return i<j; }
};

template <class T>
class generatore 
{
public:
    generatore (T  start = 0, T  stp = 1) : current(start), step(stp)
    { }
    T  operator() () { return current+=step; }
private:
    T  current;
    T  step;
};

int main () {
    vector<int> first(10);
    generate(first.begin(), first.end(), generatore<int>(10,10) );
    first.resize(first.size()*2);
    generate(first.begin()+first.size()/2, first.end(), generatore<int>(1,17) );
    cout << "\nfirst :";
    for_each (first.begin(), first.end(), print<int>);
    cout << "\nFORWARD SORT :";
    sort(first.begin(),first.end(),mystruct<int>());       // OK ! even with CLASS
    for_each (first.begin(), first.end(), print<int>);
    sort(first.begin(),first.end(),not2(mystruct<int>())); // <--- THIS LINE WILL NOT COMPILE IF I USE CLASS INSTEAD OF STRUCT
    cout << "\nBACKWARD SORT :";
    for_each (first.begin(), first.end(), print<int>);
    cout << endl;
}

Everithing , , :

struct  mystruct : binary_function<T ,T ,bool> {
    public :
    bool operator() (T  i,T  j) const { return i<j; }
};

:

g++ struct.cpp
/usr/include/c++/4.2.1/bits/stl_function.h:
'Std:: binary_negate > :
struct.cpp: 52:
/usr/include/c++/4.2.1/bits/stl_function.h:116:
error: 'typedef int std:: binary_function:: first_argument_type -
/usr/include/c++/4.2.1/bits/stl_function.h:338:
:
/usr/include/c++/4.2.1/bits/stl_function.h:119:
error: 'typedef int std:: binary_function:: second_argument_type - ....

, , ?

Answer 1

, , . struct - class public . . , public , class, :

template <class T>
class mystruct : public binary_function<T ,T ,bool> {
    public:
    bool operator() (T  i,T  j) const { return i<j; }
};

, mystruct binary_function<T,T,bool>.

, struct :

struct  mystruct : private binary_function<T ,T ,bool> {
    public: // not required here
    bool operator() (T  i,T  j) const { return i<j; }
};

class, . .

Answer 2

, ( public/private/protected)

:

class mystruct : binary_function

class mystruct : private binary_function

struct mystruct : binary_function

struct mystruct : public binary_function

mystruct binary_function, binary_function on mystruct mystruct binary_function ( ).

, mystruct , binary_function, mystruct , binary_function.

, , , binary_function, , , :

template <class _Arg1, class _Arg2, class _Result>
struct binary_function {
  typedef _Arg1 first_argument_type;   ///< the type of the first argument (no surprises here)
  typedef _Arg2 second_argument_type;  ///< the type of the second argument
  typedef _Result result_type;         ///< type of the return type
};